This invention relates to a method of reducing the iron content of a material containing calcium, silicon and oxygen, and iron in the form of a ferrite and/or oxide, and particularly to the treatment of slag, particularly to make it suitable for use as a raw material for the manufacture of a hydraulic cement.
In the manufacture of steel, slag is produced which is a waste product and which is generally characterised by the presence of significant percentages of calcium oxide, silica and iron in the form of iron oxides or iron ferrites. The actual composition of these slags depends on the raw materials used in the process and type of furnace employed. Expressing the iron as Fe.sub.2 O.sub.3, as is common in the cement industry, a typical range of the concentration of the above compounds in such a slag is:
CaO 20%-60% SiO.sub.2 14%-22% Fe.sub.2 O.sub.3 10%-60%
These slags are tapped from the furnace in which the steel is produced and are presently discarded onto waste dumps as no particular use has been found for them to date. The main problem with these slags is that they are volumetrically unstable and either break up into very fine powders (falling slags) or into small particles. Some steel slags can be used as aggregates in concrete, but the quantities thus utilised are small in comparison with the total quantity of slags produced. Further, these slag heaps are environmentally unacceptable.
These slags contain calcium oxide and silica in quantities which makes them potentially suitable for the manufacture of Portland cement. However, the iron content is too high for the slag to be used in reasonable quantities in the manufacture of such a cement.
The problem of high iron contents in Portland cement is that during the clinker burning process it causes severe coating problems and could cause high free lime contents during the cooling of the clinker. Additionally di-calcium ferrites cause durability problems in concrete, due to continued expansion after the concrete has hardened.
It has been found that the iron in some of the slags, which in cement chemistry is customarily expressed as Fe.sub.2 O.sub.3, is present in the form of fairly strongly magnetic and very weakly magnetic particles, the latter being in the form of di-calcium ferrite which is microscopically distributed throughout the slag. Thus if a very strong magnet is employed all of the slag reports to the magnetic poles and no separation of the iron compounds is possible.
The Fe.sub.2 O.sub.3 content of this type of slag can in the first instance be reduced by subjecting the material to further magnetic treatment using a magnet of medium strength of about 30-100 Gauss. This removes the free iron compounds which are strongly magnetic.
In other slags, the iron is present also in the form of FeO which is evenly distributed throughout the slag. No separation is possible by magnetic treatment of this type of slag.
In French Patent No 73.05145 a method is described in which bauxite and carbon are added to molten slag at a temperature of about 1 700.degree. C. The carbon is clearly added to reduce the iron to the metallic form. Apart from the high energy requirements, this method appears to have practical problems since the mixing of a liquid at 1 700.degree. C. with cold solids on an industrial scale may be very difficult.
Netherlands Patent No 7712077 describes the processing of LD slag from an oxysteel production process which is carried out by (a) heating a mixture of the slag, carbon and an Al.sub.2).sub.3 and SiO.sub.2 containing flux, to a temperature above 1 350.degree. C. and up to 1 400.degree. C. to reduce the metal oxides in the slag, and (b) cooling the mixture to 200.degree. C. and separating iron from the resulting fine powder. The amount of flux added in such that the CaO/Al.sub.2 O.sub.3 /SiO.sub.2 composition in weight percent falls within a zone in the phase diagram at 1 350.degree. C. defined by the points (1) 55/3/42, (2) 60/28/12, (3) 70/4/26.
On cooling, the mixture disintegrated spontaneously into fine slag particles and relatively coarse iron particles which are separated by magnetic separation or air sifting. The residual slag particles may be reacted with lime to produce Portland cement. The process is carried out in a rotary kiln. This process suffers from the disadvantages that it requires a significant energy input and that it may be impractical to carry out the process in a rotary kiln because of the formation of low melting point compounds which would give rise to build-up problems.